Usually, the first step in image processing is to correct the exposure. A common way of doing this is by adjusting levels – i.e. fitting the histogram boundaries to the tonal range of the image. The tonal range for an image is 0-255, but pixels of not all tones may be present in the image. Fitting the histogram to actual tonal range improves the exposure. This is done in Colors > Levels, by either dragging the extreme black tone to the leftmost boundary of the histogram (Fig. 1), or the extreme white tone to the rightmost boundary of the histogram if these tonal extremes do not match the histogram extremes.
A. Over-exposed image
The exposure of such an image must be reduced to acceptable limits (subjective to the photographer). A very simple way to do this is by using layer merge in Multiply mode to merge two copies of the original image. Open the original image to be adjusted and duplicate the default layer (Layer > Duplicate Layer, or Layer > New from visible). Select the top layer and change the layer-merge mode from Normal to Multiply in the drop-down. At 100% opacity, it gives a fairly acceptable exposure reduction without compromising details (Fig. 2). If more exposure is desired, the opacity may be reduced; if more correction is required, the two layers (of Fig. 2) can then be merged, the resulting single layer duplicated and the process repeated again (Fig. 3).
B. Under-exposed image
The exposure of such image must be increased to acceptable limits. The image in Fig. 3 can be considered to be an example of under-exposed image here. Similar to (A), we can correct the exposure by duplicating the image and merging the layers. Only this time we use Screen mode (Fig. 4) of layer-merge rather than Multiply. As earlier, this can be done multiple times to obtain the desired correction, and/ or opacity can be tweaked to taste at every merge.
Exposure can be further tweaked using external filters, curves, levels etc., but this gives fairly acceptable results without much effort.